My Paige is a very picky eater and will only eat cooked MEAT or wet food. She picks at kibble. But, my Phoebe has a sensitive system and cannot tolerate many wet cat foods. She can tolerate the cooked meat and kibble the best. After conversing with my veterinary school instructors, my own vet, and much research... I decided grain free is the best route to go for everyone involved.
In the wild your cat's diet would consist of primarily meat based proteins, and the occasional fruit and vegetable. Grain was consumed very little. Grain has in fact been found to cause or irritate some pets allergies. Grain is also very hard to digest especially in aged pets.
Some of the benefits of feeding grain free (not limited to):
Shiny coat, decreased allergies, healthy skin, increased appetite in picky eaters, increased energy, ease in passing of stools, reduced stools and decrease in weight for obese cats.
Paige LOVES the kibble now.. I am hoping she'll start putting on weight. I feel she is too thin. Phoebe's sensitive system is not bothered at all.
Wellness(core) & Taste of the Wild seem to be their favorites so far!
We don't have one who sell Wellness Core here.. I would love to try it too! 
) but the smell is drastically decrease the time I switch his food. Because when he arrived last December, he's just been fed Friski*s, & the stool is hard, very dark in color & very smelly. While now, his stool is softer, not so dark (good indication because if you feed them raw/cooked meat, the stool color will be brownish light, so this means the food is good for him), and smells great (great if compare to before! 
